1. Field of the Invention
The present invention relates to plasma generators, and more particularly to devices having a nozzle that discharges a plasma plume which can be generated using microwaves.
2. Discussion of the Related Art
In recent years, the progress on producing plasma has been increasing. Typically, plasma consists of positive charged ions, neutral species and electrons. In general, plasmas may be subdivided into two categories: thermal equilibrium and thermal non-equilibrium plasmas. Thermal equilibrium implies that the temperature of all species including positive charged ions, neutral species, and electrons, is the same.
Plasmas may also be classified into local thermal equilibrium (LTE) and non-LTE plasmas, where this subdivision is typically related to the pressure of the plasmas. The term “local thermal equilibrium (LTE)” refers to a thermodynamic state where the temperatures of all of the plasma species are the same in the localized areas in the plasma.
A high plasma pressure induces a large number of collisions per unit time interval in the plasma, leading to sufficient energy exchange between the species comprising the plasma, and this leads to an equal temperature for the plasma species. A low plasma pressure, on the other hand, may yield one or more temperatures for the plasma species due to insufficient collisions between the species of the plasma.
In non-LTE, or simply non-thermal plasmas, the temperature of the ions and the neutral species is usually less than 100° C., while the temperature of electrons can be up to several tens of thousand degrees in Celsius. Therefore, non-LTE plasma may serve as highly reactive tools for powerful and also gentle applications without consuming a large amount of energy. This “hot coolness” allows a variety of processing possibilities and economic opportunities for various applications. Powerful applications include metal deposition system and plasma cutters, and gentle applications include plasma surface cleaning systems and plasma displays.
One of these applications is plasma sterilization, which uses plasma to destroy microbial life, including highly resistant bacterial endospores. Sterilization is a critical step in ensuring the safety of medical and dental devices, materials, and fabrics for final use. Existing sterilization methods used in hospitals and industries include autoclaving, ethylene oxide gas (EtO), dry heat, and irradiation by gamma rays or electron beams. These technologies have a number of problems that must be dealt with and overcome and these include issues as thermal sensitivity and destruction by heat, the formation of toxic byproducts, the high cost of operation, and the inefficiencies in the overall cycle duration. Consequently, healthcare agencies and industries have long needed a sterilizing technique that could function near room temperature and with much shorter times without inducing structural damage to a wide range of medical materials including various heat sensitive electronic components and equipment.
These changes to new medical materials and devices have made sterilization very challenging using traditional sterilization methods. One approach has been using a low pressure plasma (or equivalently, a below-atmospheric pressure plasma) generated from hydrogen peroxide. However, due to the complexity and the high operational costs of the batch process units needed for this process, hospitals use of this technique has been limited to very specific applications. Also, low pressure plasma systems generate plasmas having radicals that are mostly responsible for detoxification and partial sterilization, and this has negative effects on the operational efficiency of the process.
It is also possible to generate an atmospheric plasma such as for treating surfaces, such as pre-treatment of plastic surfaces. One method of generating an atmospheric plasma is taught by U.S. Pat. No. 6,677,550 (Förnsel et al.). Förnsel et al. disclose a plasma nozzle in FIG. 1, where a high-frequency generator applies high voltage between a pin-shaped electrode 18 and a tubular conducting housing 10. Consequently, an electric discharge is established therebetween as a heating mechanism. Förnsel et al. as well as the other existing systems that use a high voltage AC or a Pulsed DC to induce an arc within a nozzle and/or an electric discharge to form a plasma has various efficiency drawbacks. This is because the initial plasma is generated inside the nozzle and it is guided by the narrow slits. This arrangement allows some of the active radicals to be lost inside the nozzle. It also has other problems in that this nozzle design has a high power consumption and produces a high temperature plasma.
Another method of generating an atmospheric plasma is described in U.S. Pat. No. 3,353,060 (Yamamoto et al.). Yamamoto et al disclose a high frequency discharge plasma generator where high frequency power is supplied into an appropriate discharge gas stream to cause high-frequency discharge within this gas stream. This produces a plasma flame of ionized gas at an extremely high temperature. Yamamoto et al. uses a retractable conductor rod 30 and the associated components shown in FIG. 3 to initiate plasma using a complicated mechanism. Yamamoto et al. also includes a coaxial waveguide 3 that is a conductor and forms a high-frequency power transmission path. Another drawback of this design is that the temperature of ions and neutral species in the plasma ranges from 5,000 to 10,000° C., which is not useful for sterilization since these temperatures can easily damage the articles to be sterilized.
Using microwaves is one of the conventional methods for generating plasma. However, existing microwave techniques generate plasmas that are not suitable, or at best, highly inefficient for sterilization due to one or more of the following drawbacks: their high plasma temperature, a low energy field of the plasma, a high operational cost, a lengthy turnaround time for sterilization, a high initial cost for the device, or they use a low pressure (typically below atmospheric pressure) using vacuum systems. Thus, there is a need for a sterilization system that: 1) is cheaper than currently available sterilization systems, 2) uses nozzles that generate a relatively cool plasma and 3) operates at atmospheric pressure so no vacuum equipment is needed.